Fix namespace in changelog entry for 2.0.0

[herndlm]

Quite late, I know, but I just upgraded Guzzle 1 -> 2 in an app and noticed errors about undefined functions. When going through the changelogs at the same time I thought "they must be still there, since only Guzzle\Psr7 functions were removed" 😅

I thought then that it might make sense to still adapt this in case other people check old breaking changes (in the current branch's CHANGELOG at least) and make a similar mistake.

[herndlm]

~~btw were all functions moved to Utils? If so, should this be mentioned too maybe? I could just add it.~~ never mind, apparently they weren't

[GrahamCampbell]

Thanks.